In a steel rolling mill, for example, a spider joint is disposed at some place of a driving shaft connected between a mill roll and a drive motor in order to carry out a rolling process as allowing the mill roll to vertically move relative to a steel material being rolled.
The above spider joint includes one which has the following constitution as disclosed in Japanese Unexamined Patent Publication No. H11(1999)-51073, for example. The spider joint is constituted such that a bearing cup is provided on each of four shafts of a cross shaft and rollers are rollably interposed between the cup and the shaft so as to utilize an outer periphery of the shaft as a rolling contact surface. This spider joint has the respective pairs of bearing cups on different lines connected to respective ends of a driving shaft portion and a driven shaft portion respectively disposed on a drive-motor-side and a mill-roll side of the driving shaft, whereby the joint is incorporated in the driving shaft for transmitting a torque of the drive motor to the mill roll side.
By the way, the aforementioned spider joint tends to suffer flaking on a shaft surface according to the length of operation time because the mill roll applies quite a great load to the shafts and bearing cups during the steel rolling process. What is more, the vertical motion of the mill roll causes the bearing cups to pivotally move relative to the shafts on which the bearing cups are mounted. Hence, the rolling contact surface provided on each of the shafts is susceptible to a partial damage such as flaking or cracks, which is produced in a circumferential direction thereof. In cases, the four shafts may suffer different degrees of flaking on a shaft surface.
In the rolling mill or the like, the cross shaft of the spider joint has an axially peripheral portion covered by the four bearing cups, and also has a central portion thereof substantially closely connected with the respective ends of the driving shaft portion and the driven shaft portion. Thus, the spider joint with the cross shaft substantially unexposed to outside is incorporated in the driving shaft. This makes it difficult for the spider joint to be mounted with a sensor for detecting the damage, such as flaking, on the rolling contact surface provided on each of the shafts. Hence, the detection of the damage based on the results of sensor detection is also impracticable. It is therefore required to subject the rolling mill to a periodical inspection, wherein the spider joint is disassembled from the driving shaft and is totally disassembled by dismounting the bearing cups from the shafts, so that the individual shafts may be visually inspected for flaking. This periodical inspection operation takes an enormous quantity of labor and time. In another method, an iron content of spent grease is determined so as to analogize the occurrence of damage on the rolling contact surface of the shaft. The grease is supplied to the rolling contact surface. In this method, however, the detection of damage may not be carried out at any one given point in time. In addition, the detection of damage based on the analogy has poor accuracies. What is more, this method is not adapted for the detection of damage on a per-shaft basis.
It is therefore an object of the invention to provide novel technical means for providing a high-accuracy and early detection of damage, such as flaking on the rolling contact surface provided on the respective shafts of the cross shaft of the spider joint incorporated in the driving shaft of the rolling mill or the like, on a per-shaft basis.